This invention relates to catalytic transfer hydrogenation, particularly in the presence of a complexed transition metal, to a catalyst for such hydrogenation and to a process of making optically active compounds.
Hydrogenation by hydrogen transfer using catalysts containing phosphorus- or nitrogen- ligands was reviewed at length by Zassinovich et al. in Chem. Rev., 1992, 92, 1051-1069. These authors concluded xe2x80x98in spite of excellent achievements, much work remains to be donexe2x80x99.
Transfer hydrogenation using catalysts in which the transition metal is coordinated to a benzenoid hydrocarbon have been explored. The following publications are of interest:
(1) Noyori et al., J. A. C. S., 1995, 117, 7562-7563: which discloses that use of chloro-ruthenium-mesitylene-N-monotosyl-1,2-diphenylethylenediamine as catalyst in the transfer hydrogenation of acetophenone to 1-phenylethanol by propan-2-ol gave up to a 95% yield of product having 97% enantiomeric excess. Similar results were obtained starting from other alkylaryl ketones. The efficiency of corresponding catalyst containing benzene in place of mesitylene was more sensitive to substituents on the aryl group of the starting ketone. Reaction times were generally rather long, typically 15h; at longer reaction times stereoselectivity decreased, apparently owing to reverse hydrogenation. No turnover numbers are reported. The authors commented that xe2x80x98the overall catalytic performance is unable to rival that of the current best hydrogenation methodxe2x80x99 as described in an earlier publication by themselves.
(2) Noyori et al., J. Chem. Soc. Chem. Commun., 1996, 233-234: which discloses that catalysts similar to those of Noyori et al. (1) above but containing other alkylbenzene ligands and various beta-amino alcohols in place of the diphenylethylenediamine were to differing extents effective in the hydrogenation of acetophenone. The beta-amino alcohol ligand gave greater catalyst stability. The preferred arene ligand was hexamethylbenzene. Turnover numbers were up to 227 moles of product per mole of catalyst per hour.
(3) Noyori et al., J. A. C. S., 1996, 118, 2521-2522: which discloses that to prevent reverse hydrogen transfer in the process of Noyori et al. (1) above, formic acid-triethylamine was used as hydrogen source. Reaction times mainly over 14h and up to 90h were used; no turnover numbers are reported.
(4) Noyori et al., J. A. C. S., 1996, 118, 4916-4917: which discloses that the process of Noyori et al. (3) above is effective for reduction of imines (especially cyclic imines) to enantioselected amines.
These processes appear to require relatively long cycle times. As well as involving uneconomic utilisation of chemical plant, such slow reaction can lead to decomposition of the catalytic complex and slow loss of product optical purity; also it affords limited scope for adjusting reaction conditions such as temperature and reactant concentration to maximise the difference in rate between enantiomerically wanted and unwanted reactions.
Besides the phosphorus-, nitrogen- and benzene-ligated transition metals, complexes based on pentamethylcyclopentadienyl (hereinafter Cp*) have been shown to be effective as catalysts in homogeneous hydrogenation of olefins by free hydrogen (Maitlis, Acc. Chem. Res., 1978, 11, 301-307; Maitlis et al., J. Chem. Soc. Dalton, 1978, 617-626); there was no disclosure of hydrogen transfer in the absence of free hydrogen or of catalysts containing a chelating or chiral-directing ligand.
Complexes of iridium with Cp* and acylated or sulphonylated alpha amino carboxylic acid have been described by Grotjahn et al. (J. A. C. S., 1994, 116, 6969-6970) but without evidence of catalytic activity. Complexes of rhodium with Cp* and 2,2,-bipyridyls and the use of these with formate to hydrogenate nicotinamide adenine dinucleotide (NAD) to NADH have been described by Steckhan et al. (Angew. Chem. Int. Ed. Engl., 1990, 29(4), 388-390). Turnover frequencies up to 67.5 per h are reported, but no activity after 100 catalytic cycles.
We have now found that stereoselective transfer hydrogenation can be efficiently carried out by means of a catalyst comprising a complex of a transition metal, a chelating ligand and a cyclopentadienyl group.
According to a first aspect of the present invention there is provided a process for the transfer hydrogenation of a compound of formula (1) to produce a compound of formula (2) 
wherein:
X represents CR3R4, NR5, (NR5R6)+Qxe2x88x92, O or S;
R1, R2, R3, R4, R5 and R6 each independently represents a hydrogen atom, an optionally substituted hydrocarbyl, a perhalogenated hydrocarbyl or an optionally substituted heterocyclyl group, one or more of R1 and R2, R1 and R3, R2 and R4, R3 and R4, R1 and R5, R2 and R6 and R5 and R6 optionally being linked in such a way as to form an optionally substituted ring(s); and
Qxe2x88x92 represents an anion;
said process comprising reacting the compound of formula (1) with a hydrogen donor in the presence of a catalyst, characterised in that the catalyst has the general formula: 
wherein:
R7 represents an optionally substituted cyclopentadienyl group;
A represents xe2x80x94NR8xe2x80x94, xe2x80x94NR9xe2x80x94, xe2x80x94NHR8 or xe2x80x94NR8R9 where R8 is H, C(O)R10, SO2R10, C(O)NR10R14, C(S)NR10R14, C(xe2x95x90NR14)SR15 or C(xe2x95x90NR14)OR15, R9 and R10 each independently represents an optionally substituted hydrocarbyl, perhalogenated hydrocarbyl or an optionally substituted heterocyclyl group, and R14 and R15 are each independently hydrogen or a group as defined for R10;
B represents xe2x80x94Oxe2x80x94, xe2x80x94OH, OR11, xe2x80x94Sxe2x80x94, xe2x80x94SH, SR11, xe2x80x94NR11xe2x80x94, xe2x80x94NR12xe2x80x94, xe2x80x94NHR12 or xe2x80x94NR11R12 where R12 is H, C(O)R13, SO2R13, C(O)NR13R16, C(S)NR13R16, C(xe2x95x90NR16)SR17 or C(xe2x95x90NR16)OR17, R11 and R13 each independently represents an optionally substituted hydrocarbyl, perhalogenated hydrocarbyl or an optionally substituted heterocyclyl group, and R16 and R17 are each independently hydrogen or a group as defined for R13;
E represents a linking group;
M represents a metal capable of catalysing transfer hydrogenation; and
Y represents an anionic group, a basic ligand or a vacant site;
provided that when Y is not a vacant site that at least one of A or B carries a hydrogen atom.
The catalytic species is believed to be substantially as represented in the above formula. It may be introduced on a solid support.
Hydrocarbyl groups which may be represented by R1-6, R9, R10, R11 and R13-17 independently include alkyl, alkenyl and aryl groups, and any combination thereof, such as aralkyl and alkaryl, for example benzyl groups.
Alkyl groups which may be represented by R1-6, R9, R10, R11 and R13-17 include linear and branched alkyl groups comprising up to 20 carbon atoms, particularly from 1 to 7 carbon atoms and preferably from 1 to 5 carbon atoms. When the alkyl groups are branched, the groups often comprising up to 10 branch chain carbon atoms, preferably up to 4 branch chain atoms. In certain embodiments, the alkyl group may be cyclic, commonly comprising from 3 to 10 carbon atoms in the largest ring and optionally featuring one or more bridging rings. Examples of alkyl groups which may be represented by R1-6, R9, R10, R11 and R13-17 include methyl, ethyl, propyl, 2-propyl, butyl, 2-butyl, t-butyl and cyclohexyl groups.
Alkenyl groups which may be represented by R1-6, R9, R10, R11 and R13-17 include C2-20, and preferably C2-6 alkenyl groups. One or more carbon-carbon double bonds may be present. The alkenyl group may carry one or more substituents, particularly phenyl substituents. Examples of alkenyl groups include vinyl, styryl and indenyl groups. When either of R1 or R2 represents an alkenyl group, a carbon-carbon double bond is preferably located at the position xcex2 to the Cxe2x95x90X moiety. When either of R1 or R2 represents an alkenyl group, the compound of formula (1) is preferably an xcex1,xcex2-unsaturated ketone.
Aryl groups which may be represented by R1-6, R9, R10, R11 and R13-17 may contain 1 ring or 2 or more fused rings which may include cycloalkyl, aryl or heterocyclic rings. Examples of aryl groups which may be represented by R1-6, R9, R10, R11 and R13-17 include phenyl, tolyl, fluorophenyl, chlorophenyl, bromophenyl, trifluoromethylphenyl, anisyl, naphthyl and ferrocenyl groups.
Perhalogenated hydrocarbyl groups which may be represented by R1-6, R9, R10, R11 and R13-17 independently include perhalogenated alkyl and aryl groups, and any combination thereof, such as aralkyl and alkaryl groups. Examples of perhalogenated alkyl groups which may be represented by R1-6, R9, R10, R11 and R13-17 include xe2x80x94CF3 and xe2x80x94C2F5.
Heterocyclic groups which may be represented by R1-6, R9, R10, R11 and R13-17 independently include aromatic, saturated and partially unsaturated ring systems and may constitute 1 ring or 2 or more fused rings which may include cycloalkyl, aryl or heterocyclic rings. The heterocyclic group will contain at least one heterocyclic ring, the largest of which will commonly comprise from 3 to 7 ring atoms in which at least one atom is carbon and at least one atom is any of N, O, S or P. When either of R1 or R2 represents or comprises a heterocyclic group, the atom in R1 or R2 bonded to the Cxe2x95x90X group is preferably a carbon atom. Examples of heterocyclic groups which may be represented by R1-6, R9, R10, R11 and R13-17 include pyridyl, pyrimidyl, pyrrolyl, thiophenyl, furanyl, indolyl, quinolyl, isoquinolyl, imidazoyl and triazoyl groups.
When any of R1-6, R9, R10, R11 and R13-17 is a substituted hydrocarbyl or heterocyclic group, the substituent(s) should be such so as not to adversely affect the rate or stereoselectivity of the reaction. Optional substituents include halogen, cyano, nitro, hydroxy, amino, thiol, acyl, hydrocarbyl, perhalogenated hydrocarbyl, heterocyclyl, hydrocarbyloxy, mono or di-hydrocarbylamino, hydrocarbylthio, esters, carbonates, amides, sulphonyl and sulphonamido groups wherein the hydrocarbyl groups are as defined for R1 above. One or more substituents may be present.
When any of R1 and R2, R1 and R3, R2 and R4, R3 and R4, R1 and R5, R2 and R6, and R5 and R6 are linked in such a way that when taken together with either the carbon atom and/or atom X of the compound of formula (1) that a ring is formed, it is preferred that these be 5, 6 or 7 membered rings. Examples of such compounds of formula (1) include 3,4-dihydroisoquinoline, 1-tetralone, 2-tetralone, 4-chromanone, 1-methyl-6,7-dimethoxy-3,4-dihydroisoquinoline, 1-benzosubarone, 2-indanone and 1-indanone.
Compounds of formula (1) where X is represented by NR5 or (NR5R6)+Qxe2x88x92, include imines or iminium salts. Where a compound of formula (1) is an imine, it may optionally be converted to an iminium salt. Iminium salts are preferred over imines. Preferred iminium salts are represented by compounds of formula (1) in which X is (NR5R6)+Qxe2x88x92 such that either R5 or R6 are hydrogen but that R5 or R6 are not identical. When the compound of formula (1) is an iminium salt, an anion represented by Qxe2x88x92 is present. Examples of anions which may be present are halide, hydrogen sulphate, tosylate, formate, acetate, tetrafluoroborate, trifluoromethanesulphonate and trifluoroacetate.
X is most preferably O.
In certain preferred embodiments, R1 and R2 are both independently C1-6 alkyl, both independently aryl, particularly phenyl, or one is aryl, particularly phenyl and one is C1-6 alkyl. Substituents may be present, particularly substituents para to the Cxe2x95x90X group when one or both of R1 and R2 is a phenyl group.
Most advantageously, the compound of formula (1) is prochiral, such that the hydrogenated product of formula (2) comprises a chiral atom to which R1, R2 and X are each bonded. Such an asymmetric transfer hydrogenation process forms an especially preferred aspect of the present invention. Most commonly, when the compound of formula (1) is prochiral, R1 and R2 are different, and neither is hydrogen. Usefully, one of R1 and R2 is aliphatic and the other is aryl or heterocyclyl.
Examples of compounds of formula (1) include acetophenone, 4-chloroacetophenone, 4-methoxyacetophenone, 4-trifluoromethylacetophenone, 4-nitroacetophenone, 2-chloroacetophenone and acetophenone benzylimine.
Hydrogen donors include hydrogen, primary and secondary alcohols, primary and secondary amines, carboxylic acids and their esters and amine salts, readily dehydrogenatable hydrocarbons, clean reducing agents, and any combination thereof.
Primary and secondary alcohols which may be employed as hydrogen donors comprise commonly from 1 to 10 carbon atoms, preferably from 2 to 7 carbon atoms, and more preferably 3 or 4 carbon atoms. Examples of primary and secondary alcohols which may be represented as hydrogen donors include methanol, ethanol, propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol, cyclopentanol, cyclohexanol, benzylalcohol, and menthol. When the hydrogen donor is an alcohol, secondary alcohols are preferred, especially propan-2-ol and butan-2-ol.
Primary and secondary amines which may be employed as hydrogen donors comprise commonly from 1 to 20 carbon atoms, preferably from 2 to 14 carbon atoms, and more preferably 3 or 8 carbon atoms. Examples of primary and secondary amines which may be represented as hydrogen donors include ethylamine, propylamine, isopropylamine, butylamine, isobutylamine, hexylamine, diethylamine, dipropylamine, di-isopropylamine, dibutylamine, di-isobutylamine, dihexylamine, benzylamine, dibenzylamine and piperidine. When the hydrogen donor is an amine, primary amines are preferred, especially primary amines comprising a secondary alkyl group, particularly isopropylamine and isobutylamine.
Carboxylic acids or their esters which may be employed as hydrogen donors comprise commonly from 1 to 10 carbon atoms, preferably from 1 to 3 carbon atoms. In certain embodiments, the carboxylic acid is advantageously a beta-hydroxy-carboxylic acid. Esters may be derived from the carboxylic acid and a C1-10 alcohol. Examples of carboxylic acids which may be employed as hydrogen donors include formic acid, lactic acid, ascorbic acid and mandelic acid. When a carboxylic acid is employed as hydrogen donor, at least some of the carboxylic acid is preferably present as an amine salt or ammonium salt. Amines which may be used to form such salts include both aromatic and non-aromatic amines, also primary, secondary and tertiary amines and comprise typically from 1 to 20 carbon atoms. Tertiary amines, especially trialkylamines, are preferred. Examples of amines which may be used to form salts include trimethylamine, triethylamine, di-isopropylethylamine and pyridine. The most preferred amine is triethylamine. When at least some of the carboxylic acid is present as an amine salt, particularly when a mixture of formic acid and triethylamine is employed, the mole ratio of acid to amine is commonly about 5:2. This ratio may be maintained during the course of the reaction by the addition of either component, but usually by the addition of the carboxylic acid.
Readily dehydrogenatable hydrocarbons which may be employed as hydrogen donors comprise hydrocarbons which have a propensity to aromatise or hydrocarbons which have a propensity to form highly conjugated systems. Examples of readily dehydrogenatable hydrocarbons which may be employed by as hydrogen donors include cyclohexadiene, cyclohexene, tetralin, dihydrofuran and terpenes.
Clean reducing agents which may be represented as hydrogen donors comprise reducing agents with a high reduction potential, particularly those having a reduction potential relative to the standard hydrogen electrode of greater than about xe2x88x920.1 eV, often greater than about xe2x88x920.5 eV, and preferably greater than about xe2x88x921 eV. Examples of clean reducing agents which may be represented as hydrogen donors include hydrazine and hydroxylamine.
The most preferred hydrogen donors are propan-2-ol, butan-2-ol, triethylammonium formate and a mixture of triethylammonium formate and formic acid.
The optionally substituted cyclopentadienyl group which may be represented by R7 includes cyclopentadienyl groups capable of eta-5 bonding. The cyclopentadienyl group is often substituted with from 1 to 5 hydrocarbyl groups, preferably with 3 to 5 hydrocarbyl groups and more preferably with 5 hydrocarbyl groups. Preferred hydrocarbyl substituents include methyl, ethyl and phenyl. When the hydrocarbyl substituents contain enantiomeric and/or diastereomeric centres, it is preferred that the enantiomerically and/or diastereomerically purified forms of these are used. Examples of optionally substituted cyclopentadienyl groups include cyclopentadienyl, pentamethyl-cyclopentadienyl, pentaphenylcyclopentadienyl, tetraphenylcyclopentadienyl, ethyltetramethylpentadienyl, menthyltetraphenylcyclopentadienyl, neomenthyl-tetraphenylcyclopentadienyl, menthylcyclopentadienyl, neomenthylcyclopentadienyl, tetrahydroindenyl, menthyltetrahydroindenyl and neomenthyltetrahydroindenyl groups. Pentamethylcyclopentadienyl is especially preferred.
When either A or B is an amide group represented by xe2x80x94NR8xe2x80x94, xe2x80x94NHR8, NR8R9, xe2x80x94NR12xe2x80x94, xe2x80x94NHR12 or NR11R12 wherein R9 and R11 are as hereinbefore defined, and where R8 or R12 is an acyl group represented by xe2x80x94C(O)R10 or xe2x80x94C(O)R13, R10 and R13 independently are often linear or branched C1-7alkyl, C1-8-cycloalkyl or aryl, for example phenyl. Examples of acyl groups which may be represented by R8 or R12 include benzoyl, acetyl and halogenoacetyl, especially trifluoroacetyl, groups.
When either A or B is present as a sulphonamide group represented by xe2x80x94NR8xe2x80x94, xe2x80x94NHR8, NR8R9, xe2x80x94NR12xe2x80x94, xe2x80x94NHR12 or NR11R12 wherein R9 and R11 are as hereinbefore defined, and where R8 or R12 is a sulphonyl group represented by xe2x80x94S(O)2R10 or xe2x80x94S(O)2R13, R10 and R13 independently are often linear or branched C1-8alkyl, C1-8cycloalkyl or aryl, for example phenyl. Preferred sulphonyl groups include methanesulphonyl, trifluoromethanesulphonyl and especially p-toluenesulphonyl groups.
When either of A or B is present as a group represented by xe2x80x94NR8xe2x80x94, xe2x80x94NHR8, NR8R9, xe2x80x94NR12xe2x80x94, xe2x80x94NHR12 or NR11R12 wherein R9 and R11 are as hereinbefore defined, and where R8 or R12 is a group represented by C(O)NR10R14, C(S)NR10R14, C(xe2x95x90NR14)SR15, C(xe2x95x90NR14)OR15, C(O)NR13R16, C(S)NR13R16, C(xe2x95x90NR16)SR17 or C(xe2x95x90NR16)OR17, R10 and R13 independently are often linear or branched C1-8alkyl, such as methyl, ethyl, isopropyl, C1-8cycloalkyl or aryl, for example phenyl, groups and R14-17 are often each independently hydrogen or linear or branched C1-8alkyl, such as methyl, ethyl, isopropyl, C1-8cycloalkyl or aryl, for example phenyl, groups.
It will be recognised that the precise nature of A and B will be determined by whether A and/or B are formally bonded to the metal or are coordinated to the metal via a lone pair of electrons.
The groups A and B are connected by a linking group E. The linking group E achieves a suitable conformation of A and B so as to allow both A and B to bond or coordinate to the metal, M. A and B are commonly linked through 2, 3 or 4 atoms. The atoms in E linking A and B may carry one or more substituents. The atoms in E, especially the atoms alpha to A or B, may be linked to A and B, in such a way as to form a heterocyclic ring, preferably a saturated ring, and particularly a 5, 6 or 7-membered ring. Such a ring may be fused to one or more other rings. Often the atoms linking A and B will be carbon atoms. Preferably, one or more of the carbon atoms linking A and B will carry substituents in addition to A or B. Substituent groups include those which may substitute R1, as defined above. Advantageously, any such substituent groups are selected to be groups which do not coordinate with the metal, M. Preferred substituents include halogen, cyano, nitro, sulphonyl, hydrocarbyl, perhalogenated hydrocarbyl and heterocyclyl groups as defined above. Most preferred substituents are C1-6 alkyl groups, and phenyl groups. Most preferably, A and B are linked by two carbon atoms, and especially an optionally substituted ethyl moiety. When A and B are linked by two carbon atoms, the two carbon atoms linking A and B may comprise part of an aromatic or aliphatic cyclic group, particularly a 5, 6 or 7-membered ring. Such a ring may be fused to one or more other such rings. Particularly preferred are embodiments in which E represents a 2 carbon atom separation and one or both of the carbon atoms carries an optionally substituted aryl group as defined above or E represents a 2 carbon atom separation which comprises a cyclopentane or cyclohexane ring, optionally fused to a phenyl ring.
E preferably comprises part of a compound having at least one stereospecific centre. Where any or all of the 2, 3 or 4 atom atoms linking A and B are substituted so as to define at least one stereospecific centre on one or more of these atoms, it is preferred that at least one of the stereospecific centres be located at the atom adjacent to either group A or B. When at least one such stereospecific centre is present, it is advantageously present in an enantiomerically purified state.
When B represents xe2x80x94Oxe2x80x94 or xe2x80x94OH, and the adjacent atom in E is carbon, it is preferred that B does not form part of a carboxylic group.
Compounds which may be represented by A-E-B, or from which A-E-B may be derived by deprotonation, are often aminoalcohols, including 4-aminoalkan-1-ols, 1-aminoalkan4-ols, 3-aminoalkan-1-ols, 1-aminoalkan-3-ols, and especially 2-aminoalkan-1-ols, 1-aminoalkan-2-ols, 3-aminoalkan-2-ols and 2-aminoalkan-3-ols, and particularly 2-aminoethanols or 3-aminopropanols, or are diamines, including 1,4-diaminoalkanes, 1,3-diaminoalkanes, especially 1,2- or 2,3-diaminoalkanes and particularly ethylenediamines. Further aminoalcohols that may be represented by A-E-B are 2-aminocyclopentanols and 2-aminocyclohexanols, preferably fused to a phenyl ring. Further diamines that may be represented by A-E-B are 1,2-diaminocyclopentanes and 1,2-diaminocyclohexanes, preferably fused to a phenyl ring. The amino groups may advantageously be N-tosylated. When a diamine is represented by A-E-B, preferably at least one amino group is N-tosylated. The aminoalcohols or diamines are advantageously substituted, especially on the linking group, E, by at least one alkyl group, such as a C1-4-alkyl, and particularly a methyl, group or at least one aryl group, particularly a phenyl group.
Specific examples of compounds which can be represented by A-E-B and the protonated equivalents from which they may be derived are: 
Preferably, the enantiomerically and/or diastereomerically purified forms of these are used. Examples include (1S,2R)-(+)-norephedrine, (1R,2S)-(+)-cis-1-amino-2-indanol, (1S,2R)-2-amino-1,2-diphenylethanol, (1S,2R)-(xe2x88x92)-cis-1-amino-2-indanol, N-tosyl-(1S,2R)-cis-1-amino-2-indanol, (1R,2S)-(xe2x88x92)-norephedrine, (S)-(+)-2-amino-1-phenylethanol, (1R,2S)-2-amino-1,2-diphenylethanol, (R)-(xe2x88x92)-2-pyrrolidinemethanol and (S)-(+)-2-pyrrolidinemethanol.
Metals which may be represented by M include metals which are capable of catalysing transfer hydrogenation. Preferred metals include transition metals, more preferably the metals in Group VIII of the Periodic Table, especially ruthenium, rhodium or iridium. When the metal is ruthenium it is preferably present in valence state II. When the metal is rhodium or iridium it is preferably present in valence state III.
Anionic groups which may be represented by Y include hydride, hydroxy, hydrocarbyloxy, hydrocarbylamino and halogen groups. Preferably when a halogen is represented by Y, the halogen is chloride. When a hydrocarbyloxy or hydrocarbylamino group is represented by Y, the group may be derived from the deprotonation of the hydrogen donor utilised in the reaction.
Basic ligands which may be represented by Y include water, C1-4 alcohols, C1-8 primary or secondary amines, or the hydrogen donor which is present in the reaction system. A preferred basic ligand represented by Y is water.
Most preferably, the nature of A-E-B, R7 and Y are chosen such that the catalyst is chiral. When such is the case, an enantiomerically and/or diastereomerically purified form is preferably employed. Such catalysts are most advantageously employed in asymmetric transfer hydrogenation processes. In many embodiments, the chirality of the catalyst is derived from the nature of A-E-B.
The process is carried out preferably in the presence of a base, especially when Y is not a vacant site. The pKa of the base is preferably at least 8.0, especially at least 10.0. Convenient bases are the hydroxides, alkoxides and carbonates of alkali metals; tertiary amines and quaternary ammonium compounds. Preferred bases are sodium 2-propoxide and triethylamine. When the hydrogen donor is not an acid, the quantity of base used can be up to 5.0, commonly up to 3.0, often up to 2.5 and especially in the range 1.0 to 3.5, by moles of the catalyst. The substantial excesses of base used by Noyori et al. appear to be unnecessary. When the hydrogen donor is an acid, the catalyst is preferably contacted with a base prior to the introduction of the hydrogen donor. In such a case, the mole ratio of base to catalyst prior to the introduction of the hydrogen donor is often from 1:1 to 3:1, and preferably about 1:1.
Although gaseous hydrogen may be present, the process is normally operated in the absence of gaseous hydrogen since it appears to be unnecessary.
The absence of oxygen is not essential. This has been shown by carrying out the process with sparging of the reactor mixture with pure oxygen: the initial turnover rate was 500 hxe2x88x921 and in 2 hours a 40% conversion was obtained. However better results have been obtained under an inert atmosphere, initial rates for example of 1080 hxe2x88x921 in static nitrogen and 1500 hxe2x88x921 with nitrogen sparging. Advantageously, the process is carried out in the substantial absence of carbon dioxide.
When the product(s) from dehydrogenation of the hydrogen donor is volatile, for example boils at under 100xc2x0 C., the removal of this volatile product is preferred. The removal can be accomplished by the use of inert gas sparging. More preferably, the removal is accomplished by distillation preferably at less than atmospheric pressure. When reduced pressure distillation is employed, the pressure is often no more than 500 mmHg, commonly no more than 200 mmHg, preferably in the range of from 5 to 100 mmHg, and most preferably from 10 to 80 mmHg.
Suitably the process is carried out at temperatures in the range of from minus 78 to plus 150xc2x0 C., preferably from minus 20 to plus 110xc2x0 C. and more preferably from minus 5 to plus 60xc2x0 C. The initial concentration of the substrate, a compound of formula (1), is suitably in the range 0.05 to 1.0 and, for convenient larger scale operation, can be for example up to 6.0 more especially 0.75 to 2.0, on a molar basis. The molar ratio of the substrate to catalyst is suitably no less than 50:1 and can be up to 50000:1, preferably between 250:1 and 5000:1 and more preferably between 500:1 and 2500:1. The hydrogen donor is preferably employed in a molar excess over the substrate, especially from 5 to 20 fold or, if convenience permits, greater, for example up to 500 fold. Reaction times are typically in the range of from 1.0 min to 24 h, especially up to 8 h and conveniently about 3 h. It appears that substantially shorter times than those disclosed in the above-mentioned publications are made practicable by the invention. After reaction, the mixture is worked up by standard procedures. A reaction solvent may be present, for example acetonitrile or, conveniently, the hydrogen donor when the hydrogen donor is liquid at the reaction temperature, particularly when the hydrogen donor is a primary or secondary alcohol or a primary or secondary amine. Usually it is preferred to operate in substantial absence of water, but water does not appear to inhibit the reaction. If the hydrogen donor or the reaction solvent is not miscible with water and the desired product is water soluble, it may be desirable to have water present as a second phase extracting the product, pushing the equilibrium and preventing loss of product optical purity as the reaction proceeds. The concentration of substrate may be chosen to optimise reaction time, yield and enantiomeric excess.
According to a second embodiment of the present invention there is provided a catalyst of general formula: 
wherein:
R7 represents an optionally substituted cyclopentadienyl group;
A represents xe2x80x94NR8xe2x80x94, xe2x80x94NR9xe2x80x94, xe2x80x94NHR8, or xe2x80x94NR8R9 where R8 is H, C(O)R10, SO2R10, C(O)NR10R14, C(S)NR10R14, C(xe2x95x90NR14)SR15 or C(xe2x95x90NR14)OR15, R9 and R10 each independently represents an optionally substituted hydrocarbyl, perhalogenated hydrocarbyl or an optionally substituted heterocyclyl group, and R14 and R15 are each independently hydrogen or a group as defined for R10;
B represents xe2x80x94Oxe2x80x94, xe2x80x94OH, OR11, xe2x80x94Sxe2x80x94, xe2x80x94SH, SR11, xe2x80x94NR11xe2x80x94, xe2x80x94NR12xe2x80x94, xe2x80x94NHR12, or xe2x80x94NR11R12 where R12 is H, C(O)R13, SO2R13, C(O)NR13R16, C(S)NR13R16, C(xe2x95x90NR16)SR17 or C(xe2x95x90NR16)OR17, R11 and R13 each independently represents an optionally substituted hydrocarbyl, perhalogenated hydrocarbyl or an optionally substituted heterocyclyl group, and R16 and R17 are each independently hydrogen or a group as defined for R13;
E represents a linking group;
M represents a metal capable of catalysing transfer hydrogenation; and
Y represents an anionic group, a basic ligand or a vacant site;
provided that (i) when Y is not a vacant site that at least one of A or B carries a hydrogen atom and (ii) when B represents xe2x80x94Oxe2x80x94 or xe2x80x94OH, that B is not part of a carboxylate group.
In the catalysts according to the present invention, A, E, B, M, R7 and Y can be as described above for the transfer hydrogenation process.
The catalytic species is believed to be substantially as represented in the above formula. It may be employed as an oligomer or metathesis product, on a solid support or may be generated in situ.
In certain embodiments it has advantageously been found that certain catalysts are preferred in the transfer hydrogenation of certain substrate types. Catalysts in which A-E-B is derived from aminoalcohols, particularly norephedrine and cis-aminoindanol, are preferred in the transfer hydrogenation of aldehydes and ketones to give alcohols. Especially, M is also rhodium (III) and R7 represents pentamethylcyclopentadienyl. Further, isopropanol is preferably employed as hydrogen donor and sodium isopropoxide is employed as a base. Catalysts in which A-E-B is derived from N-tosyldiamines are preferred in transfer hydrogenation reactions of imines and iminium salts. Especially, M is also rhodium (III) and R7 represents pentamethylcyclopentadienyl. Further, sodium isopropoxide or triethylamine are often employed as a base. When the compound of formula (1) is an imine, a mixture of formic acid and triethylamine is preferably employed as hydrogen donor and when the compound of formula (1) is a preformed imminium salt, preferably a trifluoroacetate salt, isopropanol is preferably employed as hydrogen donor.
The catalyst can be made by reacting a metal cyclopentadienyl halide complex with a compound of formula A-E-B as defined above or a protonated equivalent from which it may be derived, and, where Y represents a vacant site, reacting the product thereof with a base. The metal cyclopentadienyl halide complex preferably has the formula [MR7Z2]2, wherein M and R7 are as defined above, and Z represents a halide, particularly chloride.
For the preparation of the catalysts according to the present invention, a solvent is preferably present. Suitable reaction temperatures are in the range 0-100, for example 20-70, xc2x0 C., often giving reaction times of 0.5-5.0 h. After reaction is complete, the catalyst may if desired be isolated, but is more conveniently stored as the solution or used soon after preparation. The solution can contain the hydrogen donor and this, if a secondary alcohol, may be present in or used as the solvent for steps (a) and/or (b). The preparation and after-handling should preferably be under an inert atmosphere, and particularly in carbon dioxide and oxygen-free conditions.
The catalyst or catalyst solution is generally treated with base either just prior to use in a transfer hydrogenation reaction, or during use. This can be accomplished by adding base to the catalyst in solution, or to the compound of formula (1) in solution, or by addition to the transfer hydrogenation reaction.
Transfer hydrogenation can be accomplished by transferring the solution of catalyst to a solution of substrate, a compound of general formula I. Alternatively a solution of substrate can be added to a solution of catalyst. Base may be pre-added to the catalyst solution and/or the substrate solution, or can be added later. The hydrogen donor if not already present in the catalyst solution may be added to the substrate solution, or may be added to the reaction mixture.
The invention is illustrated by the following Examples.
Unless otherwise stated, % conversions and % enantiomeric excess (e.e.) were determined by GC.